terminal,renderer: convert structs to extern

Convert Coordinate in terminal/point.zig and CellSize, ScreenSize,
GridSize, and Padding in renderer/size.zig to extern structs. All
fields are already extern-compatible types, so this gives them a
guaranteed C ABI layout with no functional change.

src/input/mouse_encode.zig

@@ -69,7 +69,7 @@ pub const Event = struct {
     pos: Pos = .{},
 
     /// Mouse position in surface-space pixels.
-    pub const Pos = struct {
+    pub const Pos = extern struct {
         x: f32 = 0,
         y: f32 = 0,
     };